Reclamation of selenium rectifier cells



' Patented May 31, 1949 PATENT oFFlce'.

. ltECLAMATION OF S ELENH JM RECTIFII iR l. CELLS David W. Ran, Kirkwood, Mo., asslgnor to Vickcrs,

Incorporated, a corporation of Michigan No Drawing. Application 10, 1947.

' Serial No, 740,719

This invention relates to an improvement in the production of selenium rectifier cells and moreparticularly to a method of improving and reclaiming finished selenium rectifier cells.

Rectifier cells of this type comprise a metal base 'plate or electrodaa layer of semiconducting material, and a counter electrode of a. low- =melting alloy which is sprayed, or otherwise applied to the semiconductor. Such rectifiers are desirably characterized by having the resistance to electric current flow as small as possible in the conducting or forward direction and as large as possible in the nonconducting or reverse direction. However,the achievement of such a characteristic is often defeated because of the nonuniformity of resistance paths over the various portions of the rectifier cell. The effect of this nonuniformity is more pronounced in respect to the reverse characteristic where a few relatively low resistance paths can bring about a considerable lowering characteristic.

Furthermore; rectifier cells which possess the: desirablecharacteristics enumerated above often deteriorate upon storage or shelf-aging to the extent'that they are no longer suitable for use. It is an object of this invention, therefore, to

I provide a method of improving the reverse characteristics of finished selenium rectifier cells by the elimination of low resistance paths'in the nonconducting direction.

, A further object of thisinvention is to provide a method of reclaiming rectifier elements which .20 of the over-all i the contact between the'front electrode and the purpose of illustration rather than limitation,

5 Claims. (Cl. 175-366} '2 to almost 100 per cent be critical. ,Generally,.the solution is used at room temperature and the'dipping time may vary "from about 0.5 minute to about 3 minutes. The

dipped cells are removed from the bath and the surface moisture may be removed by an air blast or other suitable means. The cells may then be stored for a period of about 12 to-about 48 hours, after which they are electrically formed by passing direct-current in the" reverse direction for about 30 minutes.

It i believed that the beneficial efiects resulting from the practice of this invention are due" to an electrolytic action at the points of high leakage, which action effectively destroys the low resistance paths. It is postulated that this is accomplishedeither by the precipitation of an insoluble compound having a fairly high electrical resistivity, and thus tending to block ofi or insulate the leakage paths (for example, a compound of some constituent of the front electrode) or by an etching process which effectively destroys semiconductor at the points of high leakage.

It has been found that the organic amine electrolytes are very effective in enhancing the electrical characteristics of selenium rectifiers. The aliphatic amines and the hydroxy amines have been found especially desirable withgoodresults being obtained from the use of monoethylamine, diethanolamine, triethanolamine and diaminov propanol.

The following examples are set forth for the and are presented primarily to enable those skilled inthe art to betterpra'ctice'the present invention. The results are presented in the form of voltage measurements which are obtained by passing a standard direct current through. the

cell-in the reverse direction, and measuring the voltage drop across the cell. The higher values of voltage drop; indicate preferred rectifier performance. a

. Example! ing procedure were dipped in a 50 per cent-70 per cent aqueous solution of monoethylamine and finished rectifier cells are dipped in a solution of an electrolyte of an organic amine and then electrically formed by being subjected toa direct-current applied in the nonconducting direction.

In practicing the process comprising the present invention, finished rectifier cells are dipped in anelectrolyte of an organic amine wherein the concentration may vary from about 50 percent left for three minutes. The solution was maintained at room temperature. Surfacemoisture was removed by an airblast and the electrical forming process, used during the normal manufacturingvprocedure was repeated. 01? two experimental groups treated inthis way, a group of inventory cells had its average reverse characteristic improved by 89.5 per cent (from 7.6

and does not appear to Rectifier cells made by standard manufacture volts to 14.4 volts, average) while a group of production cells with an average reverse voltage of volts was improved by 24 per cent, the average drop being 12.4 volts after the dipping and reforming processes.

Example 2 then electrically formed by a direct-current applied in the reverse direction for minutes. This method was applied to cells from inventory stock with a resultant improvement of 81.5 per cent in the reverse direction characteristic (average reverse voltage increased from 7.6 volts to 13.8 volts) and to production plates, where the reverse voltage wasincreased to an average of 12.4 volts from an average of 10 volts.

Rectifier cells which have lost their desirable characteristics through aging may sometimes be temporarily improved by a reforming process, without dipping, but the recovery of electrical properties so engendered is not as complete as is eifected by this invention nor is the recovery at all permanent. The method of the present invention provides a lasting and permanent recovery of electrical properties lost due to aging.

The advantages of this method are rather apparent, being essentially economic. It enables an improved product to be manufactured and eliminates a large part of the waste which accompanies rejection of completed cells. It should also be pointed out that this method requires no equipment except that already used extensively in the manufacture of selenium rectiflers.

A further advantage has been shown in laboratory tests wherein treated plates stored at 122 F. and 99 per cent humidity have been found to retain their original properties essentially unchanged after considerable storage time. Untreated plates, similarly stored, show as much as 600 per cent increase in leakage current after 10 to 14 days. It will be understood that high values of leakage current result from low values of reverse direction resistance so that low leakage current values are indicative of good rectifier performance. The laboratory tests referred to above indicate that, for a period of a very few days, the untreated rectifier cells retain their original properties better than treated cells. However, upon continued storage or shelf-aging the untreated cells become less and less efiective until, after about nine days, a constant rate of deterioration of electrical properties is reached. On the other hand, treated cells begin to recover their lost efiectiveness after about three days of storage and finally regain all but a small per- 4 centage of their original properties and rema unaffected by further shelf-aging.

It will be apparent to those skilled in the art that various changes may be made in the methods described without departing from the scope of this invention. For example, they step of washing the dipped cells .may be omitted, the step of drying the surface moisture may be omitted, or the aging period between the dipping and the "forming processes may be dispensed with.

What is claimed is:

1. A method of treating finished selenium rectifier cells, which comprises dipping the cells in an electrolyte of the group consisting of the organic aliphatic amines and the hydroxy amines,

removing said cells from the electrolyte aging the dipped cells for a period of 12 to 48 hours, and then electrically forming the cells.

' 4. A method of treating finished selenium rectifier cells, which comprises dipping the cells in an electrolyte of an organic amine having a concentration of from about 50 per cent to almost 100 per cent, leaving the cells in the electrolyte for about 3 minutes, drying the dipped cells, aging the dried cells for a period of about 12 hours to REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,121,603 Lotz June 21, 1938 2,193,598 Lotz Mar. 12, '1940 2,362,545 Ellis et al. Nov. 14, 1944 2,375,355 Fahracus May 8, 1945, 

